Configuring Link Fragmentation and Interleaving for Multilink PPP

The Cisco IOS Link Fragmentation and Interleaving (LFI) feature uses Multilink PPP (MLP). MLP provides a method of splitting, recombining, and sequencing datagrams across multiple logical data links. MLP allows packets to be fragmented and the fragments to be sent at the same time over multiple point-to-point links to the same remote address.

This chapter describes the tasks for configuring MLP, and it includes example configurations.

To identify the hardware platform or software image information associated with a feature, use the Feature Navigator on Cisco.com to search for information about the feature or refer to the software release notes for a specific release. For more information, see the "Identifying Supported Platforms" section in the "Using Cisco IOS Software" chapter in this book.

About MLP Interleaving and Queueing for Real-Time Traffic

Interleaving on MLP allows large packets to be multilink encapsulated and fragmented into a small enough size to satisfy the delay requirements of real-time traffic; small real-time packets are not multilink encapsulated and are sent between fragments of the large packets.

The interleaving feature also provides a special transmit queue for the smaller, delay-sensitive packets, enabling them to be sent earlier than other flows.

Weighted fair queueing (WFQ) on MLP works at the packet level, not at the level of multilink fragments. Thus, if a small real-time packet gets queued behind a larger best-effort packet and no special queue has been reserved for real-time packets, the small packet will be scheduled for transmission only after all the fragments of the larger packet are scheduled for transmission.

WFQ is supported on all interfaces that support MLP, including MLP virtual access interfaces and virtual interface templates.

Fair queueing on MLP overcomes a prior restriction. Previously, fair queueing was not allowed on virtual access interfaces and virtual interface templates. Interleaving provides the delay bounds for delay-sensitive voice packets on a slow link that is used for other best-effort traffic.

Restrictions

Interleaving applies only to interfaces that can configure a multilink bundle interface. These interfaces include virtual templates, dialer interfaces, and ISDN BRI or PRI interfaces.

Multilink and fair queueing are not supported when a multilink bundle is off-loaded to a different system using Multichassis Multilink PPP (MMP). Thus, interleaving is not supported in MMP networking designs.

Note LFI on PPP over Frame Relay is not supported on Cisco IOS Release 12.1E.

Interleaving for Multilink PPP Configuration Task List

To configure MLP, perform the tasks described in the following sections. The task in the first section is required; the task in the remaining section is optional.

Note Fair queueing, which is enabled by default, must remain enabled on the interface.

To configure MLP and interleaving on a configured and operational interface or virtual interface template, use the following commands in interface configuration mode:

Command

Purpose

Step 1

Router(config-if)# ppp multilink

Enables MLP.

Step 2

Router(config-if)# ppp multilink interleave

Enables real-time packet interleaving.

Step 3

Router(config-if)# ppp multilink fragment-delaymilliseconds

(Optional) Configures a maximum fragment delay. If, for example, you want a voice stream to have a maximum bound on delay of 20 milliseconds (ms) and you specify 20 ms using this command, MLP will choose a fragment size based on the configured value.

! Keeps fragments of large packets small enough to ensure delay of 20 ms or less.

ppp multilink fragment-delay 20

dialer-list 1 protocol ip permit

The following is an example of interleaving statistics displayed using the show interfaces command for a particular interface on which interleaving is enabled. Interleaving data is displayed only if there are interleaves. For example, the following line shows interleaves: